Exposure means using LED array is generally known. For example, in U.S. Pat. No. 4,455,562 an LED printer is disclosed. In that printer an image is imaged line by line on a previously charged photoconductive layer by means of an array of LED's which together with part of their control electronics form part of an integrated circuit. Since not all of the LED's emit the same amount of light in response to identical electrical energization, a control circuit is disclosed which controls the pulse time of each LED separately in conjunction with the known light emission of that LED. LED's having lower light emission are energized with longer pulses, and, thus, the total emission per pulse of all the LED's is equalized.
It is known that the light emission of LED's decreases with use, therefore, the light emission of lower output LED's will decline more rapidly than that of the brighter LED's. Also, it is typical for print images to contain areas which are rarely covered with image parts, such as the edges, and areas which almost always contain image parts, e.g., the middle. LED's required to expose the edges are subjected to practically continuous loading, while LED's which have to expose the image parts are loaded less frequently. As a result of these two effects; the light emissions of the separate LED's will be subjected considerably to unequal changes over their life so that a simple aging compensation scheme becomes impossible.
Accordingly, it is an object of the invention to provide an exposure device and a printer in which all of the LED's age equally so that a simple, uniform aging compensation means is possible. The invention is based on the understanding that the LED's have to be brought in an equal state of aging in a period out of the exposure time for forming a latent image on the photosensitive belt.